A search for recombination between homologous regions of the genome will be continued in Chinese hamster ovary cells. Hybrids heterozygous at the locus for hypoxanthine phosphoribosyl transferease (hprt) will be subjected to 6-thioguanine selection for the isolation of drug-resistant segregants. Gene dosage studies of the remaining hprt mutant allele will be carried out using both genetic and biochemical schemes. These measurements will test the possibility that some segregants arise by mitotic recombination, resulting in doubling of the remaining mutant hprt allele. The limited donation of genetic information in cell fusion will be effected by fragmenting the chromosomes of one parent by 5-bromodeyuridine-plus-light treatment. Cells in which a donor hprt allele has substituted for the "recipient" gene will be selected. Such cells should be candidates for having undergone recombination and will be further tested to establish this fact. The facilitation of DNA-mediated and metaphase chromosome mediated gene transfer using liposomes will be attempted. The goal is to improve the efficiency of transfer to the point where true homologous recombinants can be obtained. Finally, mutants deficient in dihydrofolate reductase activity will be sought, for eventual use in DNA mediated transformation experiments using cloned genes.